Thiol-ene silicone additive fabrication

ABSTRACT

Methods and compositions for additive manufacturing of silicone parts are provided. The methods can use SLA printing techniques to print silicone parts that exhibit excellent hardness, tear strength and elongation at break. The parts can be produced by using low dosages of radiation. In various embodiments, the silicone compositions include a mercapto-derivatized polysiloxane having two or more functional groups, an alkenyl-derivatized polysiloxane, and a photo-initiator.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Patent ApplicationNo. 63/267,645, filed Feb. 7, 2022, which is incorporated by referencein its entirety.

FIELD OF INVENTION

The invention relates to materials and methods for additive fabrication.

BACKGROUND

Additive fabrication, or 3-dimensional (3D) printing, providestechniques for fabricating objects, typically by causing portions of abuilding material to solidify at specific locations. Additivefabrication techniques may include stereolithography (SLA), selective orfused deposition modeling, direct composite manufacturing, laminatedobject manufacturing, selective phase area deposition, multi-phase jetsolidification, ballistic particle manufacturing, particle deposition,laser sintering or combinations thereof. Many additive fabricationtechniques build parts by forming successive layers, which are typicallycross-sections of the desired object. Typically, each layer is formedsuch that it adheres to either a previously formed layer or a substrateupon which the object is built.

SUMMARY

In one aspect, an additive method of producing a three-dimensional (3D)object is provided. The method can include providing a photocurablesilicone composition in a vessel of an additive fabrication device,irradiating a layer of the photocurable silicone composition with anenergy source to form an at least partially cured layer of thephotocurable silicone composition on a build platform of the additivefabrication device or on a previously cured layer of the photocurablesilicone composition. Th photocurable silicone composition can include amercapto-derivatized polysiloxane having two or more functional groups,an alkenyl-derivatized polysiloxane, and a photo-initiator.

In another aspect, a three-dimensional object can include a photocuredsilicone composition including a crosslinked alkenyl-derivatizedpolysiloxane and a mercapto-derivatized polysiloxane having two or morefunctional groups.

In another aspect, a photo-curable resin can include a crosslinkedalkenyl-derivatized polysiloxane and a mercapto-derivatized polysiloxanehaving two or more functional groups.

In certain circumstances, providing the photocurable siliconecomposition can include extruding the photocurable silicone composition.

In certain circumstances, providing the photocurable siliconecomposition can include providing a bath of the photocurable siliconecomposition.

In certain circumstances, in the photocurable silicone composition, theratio of mercapto-derivatized polysiloxane to alkenyl-derivatizedpolysiloxane by weight can be greater than 1:99, greater than 5:95,greater than 10:90, greater than 20:80, or greater than 30:70.

In certain circumstances, in the photocurable silicone composition, theratio of mercapto-derivatized polysiloxane to alkenyl-derivatizedpolysiloxane by weight can be less than 50:50, less than 40:60, or lessthan 30:70.

The alkenyl-derivatized polysiloxane can have an average of at least twosilicon-bonded ethylenically unsaturated groups per molecule.

The photocurable silicone composition can have a ratio of sulfur atomsto vinyl groups can be between 1:1 and 3:1, or between 1.5:1 and 2.5:1,or between 1.5:1 and 5:1, or between 2.5:1 and 5:1. The photocurablesilicone composition can include a SiO₄ resin, a CH₃SiO₃ resin and/or a(CH₃)₃SiO resin.

The photocurable silicone composition can include a functionalizedsilica, alumina, fumed silica, or carbon nanotubes, or combinationsthereof.

The photocurable silicone composition can include a SiO₄ resin, aCH₃SiO₃ resin, or a (CH₃)₃SiO resin, or a combination thereof. Forexample, the mercapto-derivatized polysiloxane can include a SiO₄ resin,a CH₃SiO₃ resin, or a (CH₃)₃SiO resin, or a combination thereof. Inanother example, the alkenyl-derivatized polysiloxane can include a SiO₄resin, a CH₃SiO₃ resin, or a (CH₃)₃SiO resin, or a combination thereof.

The energy source can include at least one of UV laser, UV LCD, or UVDLP.

In certain circumstances, the laser can have a dosage of greater than 10mJ/cm², greater than 20 mJ/cm², greater than 30 mJ/cm², greater than 40mJ/cm², greater than 50 mJ/cm², greater than 60 mJ/cm², greater than 65mJ/cm², or greater than 80 mJ/cm².

In certain circumstances, the laser can provide a dosage of less than 60mJ/cm², less than 40 mJ/cm², less than 30 mJ/cm², or less than 20mJ/cm².

The mercapto-derivatized polysiloxane can have a viscosity of greaterthan 100 cP.

The alkenyl derivatized siloxane can have a viscosity of greater than400 cP.

At least partially cured layers in the shape of a three-dimensionalobject can be post-cured at a temperature between room temperature and125° C., less than 120° C., less than 110° C., less than 100° C., lessthan 90° C., less than 80° C., less than 75° C., less than 70° C., lessthan 65° C., or less than 60° C. In certain embodiments, at leastpartially cured layers in the shape of a three-dimensional object can bepost-cured at greater than 55° C. for at least 10 minutes.

The photocurable silicone composition can have a viscosity of greaterthan 500 cP, greater than 600 cP, greater than 700 cP, less than 2000cP, less than 1000 cP, greater than 5,000 cP, greater than 10,000 cP, orgreater than 15,000 cP.

The photocurable silicone composition can have a viscosity of less than500,000 cP, less than 400,000 cP, less than 300,000 cP, less than200,000 cP, less than 100,000 cP, less than 50,000 cP, less than 25,000cP, less than 20,000 cP, less than 15,000 cP, less than 10,000 cP, lessthan 5,000 cP, less than 4,000 cP, less than 3,000 cP, less than 2,000cP, less than 800 cP, less than 500 cP, or less than 300 cP.

The mercapto-derivatized siloxane can include a QT resin, a QM resin, aQDT resin, a D resin, a QTMD resin, or a QTM resin. In certaincircumstances, the mercapto-derivatized siloxane can include propylthiolgroups. In certain circumstances, the mercapto-derivatized siloxane canbe essentially devoid of methyl groups. In certain circumstances, themercapto-derivatized siloxane is not a D resin.

The alkenyl-derivatized siloxane can include a QT resin, a QM resin, aQDT resin, a D resin, a QTMD resin, or a QTM resin. In certaincircumstances, the alkenyl-derivatized can include vinyl groups oracrylate groups. In certain circumstances, the alkenyl-derivatizedsiloxane can be essentially devoid of methyl groups. In certaincircumstances, the alkenyl-derivatized siloxane is not a D resin.

The mercapto-derivatized siloxane can have an SH functionality contentof greater than 4%, greater than 8%, greater than 12%, or greater than14%.

The photocurable silicone composition can include less than 5%, lessthan 1%, less than 0.1%, or less than 0.01% silicone monomer, by weight.In certain circumstances, the photocurable silicone composition caninclude less than 5%, less than 1%, less than 0.1%, or less than 0.01%mercapto-derivatized polysiloxanes, by weight. In certain circumstances,the photocurable silicone composition can include two or more distinctmercapto-derivatized polysiloxanes. In certain circumstances, thephotocurable silicone composition can include two or morealkenyl-derivatized polysiloxanes.

In certain circumstances, the composition can include amercapto-derivatized siloxane resin wherein some or all of the methylgroups are replaced by alkylthiol groups. In other words, some or all ofthe organic substituents can be alkylthiol groups. For example, themercapto-derivatized siloxane can be essentially devoid of methylgroups.

In certain circumstances, the alkenyl-derivatized polysiloxane caninclude a plurality of vinyl groups.

In certain circumstances, the alkenyl-derivatized polysiloxane caninclude a plurality of acrylate groups.

In certain circumstances, the photocurable silicone composition caninclude a flame retardant.

In certain circumstances, the photocurable silicone composition caninclude an odor neutralization agent.

In certain circumstances, the photocurable silicone composition caninclude a scent masking agent.

A three dimensional object made using any of these compositions canexhibit a Shore A hardness of greater than 10, greater than 20, greaterthan 40, or greater than 60.

The foregoing apparatus and method embodiments may be implemented withany suitable combination of aspects, features, and acts described aboveor in further detail below. These and other aspects, embodiments, andfeatures of the present teachings can be more fully understood from thefollowing description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIG. 1 illustrates a reaction of a vinyl compound and a thiolatedcompound;

FIG. 2 provides a structural example of an alkenyl-derivatizedpolysiloxane;

FIG. 3 provides a structural example of a linear mercapto-derivatizedpolysiloxane;

FIG. 4 provides a structural example of a branched mercapto-derivatizedpolysiloxane;

FIG. 5 is a photocopy of a photograph of 1 cubic centimeter siliconecubes of one embodiment;

FIG. 6 is a photocopy of a photograph of 3D silicone objects made fromanother embodiment;

FIGS. 7A and 7B are photographs of silicone objects made usingadditional embodiments;

FIG. 8 is a photograph of two different silicone objects made usingdifferent compositions;

FIG. 9A provides a photograph of a 3D frog after printing;

FIGS. 9B and 9C are photographs of the frog of FIG. 9A afterpost-production curing; and

FIGS. 10A, 10B and 10C show lattices printed using compositions ofembodiments disclosed herein.

FIG. 11 shows ultimate tensile strength for compositions including astabilizer.

DETAILED DESCRIPTION

In one aspect, the methods disclosed herein are directed to the additivefabrication of silicone polymer objects. The methods can include the useof thiol-ene silicones to produce silicone objects using SLA methods.Previous attempts to produce silicone parts using SLA techniques havefailed due to, for example, oxygen inhibition, viscosity problems or theinability to form well-cured parts. By implementing the thiol-enechemistry techniques described herein, additive manufacturing can beused to produce well-defined, properly cured silicone objects that cantake advantage of silicone chemistry and all of the attributes of SLA.

In additive fabrication, a desired part may be fabricated by formingsuccessive layers of a source material on top of one another. Forexample, in stereolithography (SLA), a part may be fabricated bysuccessively forming thin layers of a polymer by photocuring portions ofa photopolymer liquid, typically first onto a substrate and then one ontop of another. The SLA methods described herein use a photopolymerliquid that includes a mercapto-derivatized polysiloxane having two ormore end groups and an alkenyl-derivatized polysiloxane. In the presenceof a photoinitiator, an energy source such as a UV laser can initiatecross-linking, resulting in a polymerized silicone object. As usedherein, a “thiol-ene” is a compound resulting from the addition of athiol to an ene bond, and the mercapto-derivatized polysiloxanecomponent of the composition may be referred to as a crosslinker.

In one aspect, a method for producing a three-dimensional object by anadditive process is provided. In some embodiments the additive processis stereolithography or inverted stereolithography (SLA) and can bepracticed using SLA printers such as, for example, the Form3+™ availablefrom Formlabs. In some embodiments the polymerizable siliconecomposition can be extruded, for example, using a fused depositionmodeling (FDM) printer, in which a high viscosity liquid is extrudedfrom a nozzle in an uncured state and curing each layer of a productpartially or fully with UV light. A polymerizable silicone compositioncan be catalyzed using radical initiators or chemical catalysts. In somecases the radical initiator is activated by radiation that can beprovided by, for instance, a UV source (222-450 nm) such as a laser, DLPor LCD. In particular embodiments, the source is a UV laser. Aphotocurable silicone composition can include at least two siliconepre-polymers and a photoinitiator. One or more pre-polymers can be analkenyl-derivatized polysiloxane. An alkenyl-derivatized polysiloxane isa polysiloxane that includes vinyl groups that can be reacted accordingto the mechanism shown in FIG. 1 . Another pre-polymer in thephotocurable silicone composition can be a mercapto-derivatizedpolysiloxane capable of crosslinking the alkenyl-derivatizedpolysiloxane. The photocurable composition can also include aphotoinitiator.

Three dimensional objects made from the photocurable siliconecomposition exhibit features that have not been obtainable with otherformulations. For instance, attempts at SLA printing with other siliconesystems have resulted in the highly blurred objects with poor tearstrength and elongation. By using high molecular weightmercapto-derivatized polysiloxane and high molecular weightalkenyl-derivatized polysiloxane with a photoinitiator, well-defined 3Dobjects have been printed using SLA printers. The resulting objectsexhibit reduced tackiness, improved tear strength and improvedelongation at break.

The energy source can include at least one of UV laser, UV LCD, or UVDLP.

In certain circumstances, the laser can have a dosage of greater than 10mJ/cm², greater than 20 mJ/cm², greater than 30 mJ/cm², greater than 40mJ/cm², greater than 50 mJ/cm², greater than 60 mJ/cm², greater than 65mJ/cm², or greater than 80 mJ/cm².

In certain circumstances, the laser can provide a dosage of less than 60mJ/cm², less than 40 mJ/cm², less than 30 mJ/cm², or less than 20mJ/cm².

In some cases, the objects do not require postproduction curing. Atleast partially cured layers in the shape of a three-dimensional objectcan be post-cured at a temperature between room temperature and 125° C.,less than 120° C., less than 110° C., less than 100° C., less than 90°C., less than 80° C., less than 75° C., less than 70° C., less than 65°C., or less than 60° C. In certain embodiments, at least partially curedlayers in the shape of a three-dimensional object can be post-cured atgreater than 55° C. for at least 10 minutes. Postproduction curing canbe conducted at room temperature. In certain circumstances, the objectcan be submerged in a liquid during curing. For example, the liquid canbe water or glycerine. In certain circumstances, postproduction curingcan take place in an inert atmosphere, for example, under a nitrogen orargon atmosphere.

Viscosity is measured using a 50 mm parallel plate rheometer at 10 Hz.The photocurable silicone composition can have, in various embodiments,a viscosity of greater than 500 cP, greater than 600 cP, greater than700 cP, greater than 1000 cP, greater than 2000 cP, greater than 5,000cP, greater than 10,000 cP, greater than 15,000 cP, greater than 25,000cP, greater than 40,000 cP, greater than 50,000 cP, or greater than60,000 cP. In certain circumstances, the photocurable siliconecomposition can have, in various embodiments, a viscosity of less than500,000 cP, less than 400,000 cP, less than 300,000 cP, less than200,000 cP, less than 100,000 cP, less than 60,000 cP, less than 50,000cP, less than 20,000 cP, less than 15,000 cP, less than 10,000 cP, lessthan 5,000 cP, less than 4,000 cP, less than 3,000 cP, less than 2,000cP, less than 800 cP, less than 500 cP, or less than 300 cP.

The photocured silicone composition can have a ratio ofmercapto-derivatized polysiloxane to alkenyl-derivatized polysiloxane byweight of greater than 1:99, greater than 2:98, greater than 5:95,greater than 10:90, greater than 20:80, greater than 30:70, less than50:50, less than 40:60, less than 30:70, less than 20:80, less than10:90, less than 5:95, or less than 2:98.

In the photocurable silicone composition, the ratio of sulfur atoms tovinyl (or acrylate) groups (enes) can be, for example, greater than 1:1,greater than 1.5:1, greater than 2.0:1, greater than 2.5:1, greater than3.0:1, less than 3.0:1, less than 2.5:1, less than 2.0:1, less than1.5:1, or less than 1:1.

The photocurable silicone composition can include a SiO₄ resin, aCH₃SiO₃ resin or a (CH₃)₃SiO resin, or combinations thereof.

More particularly, the polysiloxanes may include different structuralunits that are known in the art as M, D, T, and Q structures. Mcompounds include three methyl groups bonded to the silicon atom. Dcompounds include two methyl groups bonded to the silicon atom. Tcompounds include a single methyl group bonded to the silicon atom. Qcompounds include only Si—O bonds and do not include methyl or othercarbon groups. A resin can have a combination of two or more of M, D, Tand Q structures. Various resins may be combinations of these types,such as QM, QT, MDT, QDT, MTQ, MDQ, QTM, QTDM, and others. In someembodiments the alkenyl-derivatized polysiloxane can include a Q resin,M resin, or a QM resin. A VQM designation indicates a QM resinterminated with vinyl groups. As an example, the alkenyl-derivatizedpolysiloxane can include 70% vinyl terminated siloxane and 30% VQMresin. In certain embodiments, the alkenyl-derivatized polysiloxane caninclude acrylate groups. For example, the acrylate group can be amethacrylate group. In certain circumstances, the alkenyl-derivatizedpolysiloxane can be a mixture of vinyl-, acrylate- ormethacrylate-derivatized siloxane.

In other cases, D and T resins may also be included. For example, themercapto-derivatized polysiloxane can be greater than 5, greater than10, greater than 20 or greater than 30% Q with the balance being M, T, Dor a combination thereof. In some cases, the resin is a combination thatincludes both Q and M (QM), and the mercapto-derivatized polysiloxanecan be greater than 5, greater than 10, greater than 20 or greater than30% QM resin. In another set of embodiments, the resin can include QTand the ratio of Q:T can be greater than 10:90, greater than 20:80 orgreater than 30:70. In certain circumstances, the D content can be lessthan 5%, less than 1%, or 0%.

Other functional groups may replace the methyl groups and can include,for instance, vinyl groups in the case of alkenyl-derivatizedpolysiloxanes. In various embodiments, the polymerizable siliconecomposition can include greater than 5%, greater than 10%, greater than15% or greater than 20% Q resins. In these and other embodiments, theamount of M resin can be greater than 1%, greater than 5% or greaterthan 10% of the total silicone resin content. In specific embodiments,mercapto-derivatized polysiloxane can be, for example, a Q resin, an Mresin, a T resin, a D resin, or any combination of the four. As usedherein, a combination resin (e.g., QM or QT) is a single polymericstructure that contains more than one type of siloxane unit. It is notsimply a mixture of different molecular compounds.

The photoinitiator can be a compound that transforms into an activeradical (radical initiator) or ion (cationic initiator) when irradiatedwith radiation such as UV light. Nonlimiting examples of cationicinitiators include acylphosphine oxides, aminoalkylphenones,dialkoxyacetophenones, hydroxyalkylphenones, benzil ketals,dialkoxyanthracenes, and benzoin ethers. Nonlimiting examples of radicalinitiators include anthraquinones, benzophenones/amines andthioxanthones/amines. The photoinitiator can be included atconcentrations of, for example, greater than or equal to 0.1%, 0.5%,1.0%, 1.5%, 2.0% or 2.5%, by weight.

The alkenyl-derivatized polysiloxane can be a polysiloxane that includesa prevalence of alkenyl groups that react with the thiol groups of amercapto-derivatized polysiloxane via alkene hydrothiolation in thepresence of radiation and a photoinitiator. The alkenyl-derivatizedpolysiloxane can also contain methyl or other functional groups. Thealkenyl-derivatized polysiloxane can be linear, cyclical or branched orany combination thereof. In some embodiments of the photocurablesilicone composition the alkenyl-derivatized polysiloxane is branchedand can be essentially free of linear polysiloxanes. For example, theremay be less than 5%, less than 2% or less than 1% linear polysiloxanesin the photocurable composition. The average molecular weight of thealkenyl-derivatized polysiloxane can be, for example, greater than 1kDa, greater than 5 kDA, greater than 10 kDa, greater than 20 kDA, orgreater than 50 kDa. In the same and other embodiments, the polysiloxanecan exhibit a molecular weight of less than 200 kDa, less than 100 kDa,less than 70 kDa, less than 50 kDa, or less than 30 kDa. Thealkenyl-derivatized polysiloxane may account for greater than 90,greater than 80, greater than 70, greater than 60, greater than 50,greater than 40, or greater than 30 percent of the photocurable siliconecomposition, by weight.

The mercapto-derivatized polysiloxane includes thiol groups and canhave, for example, greater than 1, greater than 2, greater than 3,greater than 5, greater than 6, fewer than 10, fewer than 8, fewer than7, fewer than 6, fewer than 5, fewer than 4 or fewer than 3 thiols permolecule. The mercapto-derivatized polysiloxane can be linear orbranched. As used herein, a linear polysiloxane includes repeating unitsof SiOR₂ where R can be H, O or Cn. In certain circumstances, the thiolcontent of the mercapto-derivatized polysiloxane can be between 4 and 6%by weight. In other circumstances, the thiol content can be about 10%,which can lead to products that have high strength and hardness. Forexample, products can have a hardness of 50 A and 100% elongation atbreak. In various embodiments, n can be 0, 1, 2, 3, >0, >1, >2, or >3.Examples of linear siloxanes are provided in FIGS. 2 and 3 . A branchedpolysiloxane is a polysiloxane that includes siloxane units bound tomore than just two adjacent, linear siloxane units. Compared to a linearpolysiloxane, in a branched mercapto-derivatized polysiloxane some orall of the methyl groups have been replaced with alkyl thiol groups thatmay include 3 or more carbons per alkyl thiol group. A siloxane unit ina branched polysiloxane can include siloxane units that are bound tothree or four siloxane units, and a branched silicone must thereforeinclude at least some Q and/or T bonding. Q compounds are those bound tofour adjacent siloxane units, T compounds are those bound to threeadjacent siloxane units. Branched polysiloxanes can include combinationsof Q, T, D, and M but must always include at least some Q or T. Anexample of Q is shown in FIG. 4 . Branched compounds may be consideredto be partially cross-linked already due to the non-linear structure andare reactive to UV light in the presence of a photoinitiator.

In certain circumstances, the photocurable silicone composition caninclude a silane compound, for example a polysiloxane including Si—Hunits. The Si—H units can be less than 5%, less than 3% or less than 1%of the photocurable silicone composition. A catalyst, for example, aplatinum catalyst, can be added to the photocurable silicone compositionto assist with silane crosslinking reactions.

The molecular weight of the polysiloxane can be, for example, 3 to 4kDa, 5 to 7 kDa, 8 to 10 kDa, greater than 3 kDa, greater than 5 kDA,greater than 8 kDa, greater than 50 kDa, or less than 100 kDa. Theviscosity of the mercapto-derivatized polysiloxane can be greater than100 cP, greater than 200 cP, greater than 500 cP, greater than 1,000 cP,or greater than 5,000 cP. In other embodiments the viscosity of themercapto-derivatized polysiloxane can be less than 1,000 cP, less than500 cP, or less than 200 cP.

Additional additives may be incorporated into the photocurable siliconecomposition. For example, additives can be used to alter viscosity, toimprove tear strength, to enhance conductivity, to reduce brittling,improved tear strength, and to provide color or opacity. Variousadditives may be soluble in the silicone composition or may beemulsified, mixed or dispersed therein. For instance, functionalizedmetal oxides, such as vinyl silica, or a silicone elastomer, can beadded for improved strength. Brittling of cured material during agingcan be improved with the addition of hindered amines, such as theTinuvin light stabilizers from BASF. For example, 0.5 to 1.0 phr of ahindered amine can exhibit this improvement. Additives such as silicacan be functionalized to improve dispersibility as well as to beincorporated into the polymer. As the photocurable silicone compositionmay be stored for an extended time before it is printed, additives canbe selected, or treated, for improved dispersion stability. Additivescan include antioxidants, for example, hindered phenols. Additionaladditives include, for instance, organic and inorganic pigments,conductivity enhancers such as carbon black, carbon nanotubes, ceramics,and metals, and antimicrobial additives. The photocurable siliconecomposition can include a functionalized silica, alumina, fumed silica,or carbon nanotubes, or combinations thereof. For example, carbon blackcan be added to a composition to provide grey or black color. Carbonblack can also improve small and negative features on a cured productwhile allowing full cure of the interior of the product. For example, acomposition can have a carbon black content of about 0.05 phr.

In certain circumstances, the photocurable silicone composition caninclude a flame retardant. A flame retardant can include aluminumoxides, aluminum hydroxides, magnesium oxides, magnesium hydroxides,borates, salt hydrates, organohalogen compounds (including brominatedcompounds and chlorinated compounds, or organophosphorus compounds(including phophonate, phosphinate and phosphate compounds).

In certain circumstances, the photocurable silicone composition caninclude an odor neutralization agent. An odor neutralization agent caninclude a oxidizing agent, for example, chlorine, a chlorine oxide, acarboxylic acid, a peroxide, or ozone, or a reactive compound, such asan oxirane. The oxirane can be a mono or difunctional expoxide.

In certain circumstances, the photocurable silicone composition caninclude a scent masking agent. A scent masking agent can include apleasant smelling compound, for example, a natural oil or extract, forexample, esters, such as octyl acetate, benzaldehydes, such as vanillin,lactones, or terpenes, such as limonene.

EXAMPLES Example 1

A high molecular weight alkenyl-derivatized polysiloxane and highmolecular weight mercapto-derivatized polysiloxane were polymerized atdifferent ratios as provided in Table 1. DMS-V25 is a vinyl terminatedsilicone available from Gelest that has a molecular weight of 17.2 kDaand a viscosity of 500 cP. SMS-042 is a mercapto-derivatized siliconefrom Gelest that comprises % thiol and a viscosity of 150 cP. Inaddition, each sample includes photoinitiator or a mix ofphotoinitiators (e.g., phosphine oxide photoinitiators) sensitive to thewavelength of the curing light source and optical brightener (e.g.,Uvitex OB (BASF) 2, 5 thiophenediyl bis(5-tert-butyl-1,3-benzoxazole)).

TABLE 1 % Mercapto- % % Alkenyl derivatized Photoinitiator Samplesiloxane siloxane and Optical ID DMS-V25 SMS-042 Brightener PrintableProperties A1 50 50 2% phosphine Y Soft, stretchy oxide photoinitiator,0.05% optical brightener A2 70 30 2% phosphine Y Soft, elastic, oxidestronger than photoinitiator, A1 0.05% optical brightener A3 80 20 2%phosphine Y Soft, elastic, oxide stronger than photoinitiator, A1 0.05%optical brightener A4 90 10 2% phosphine N Very soft, oxide weak andphotoinitiator, tacky 0.05% optical brightener A5 99 1 2% phosphine NDid not oxide cure to photoinitiator, object 0.05% optical brightener

Compositions with a thiol content of about 2-3% produced a softer curedmaterial. Composition with a higher thiol content, for example, 13-17%,can produce harder cured material.

Example 2

A composition similar to those of Example 1 was made with 75% DMS-V25,25% SMS-042, 2.5% phosphine oxide photoinitiator, 0.05% opticalbrightener, and 10 phr HiSil 233 (vinyl silica). This composition had aviscosity of about 800 cP and is referred to as photocurable siliconecomposition B1. Composition B1 was used to print cubes with the Form3SLA printer at an exposure of 40 mJ/cm². 1 cubic centimeter cubes wereprinted and are shown in FIG. 5 before and after postproduction curingat 60 degrees C. for 60 minutes. The cube exhibited a Shore A hardnessof 15 after curing.

Example 3

An exposure sweep from 35 to 70 mJ/cm² was performed on composition B1and the resulting prints showed improved results at 70 mJ/cm² while allnegative features were still legible. Next, a sweep was performed over arange of 65 to 130 mJ/cm². Photographs of the prints are provided inFIG. 6 and, from left to right, show samples produced at exposures of65, 70, 85, 105 and 130 mJ/cm². The samples indicate that negativefeatures were partially or completely filled above 75 mJ/cm². A fill andskin of 70 mJ/cm² was chosen as providing a suitable print forcomposition B1. Additional objects were printed using composition B1.FIG. 7A is a picture of a printed relief map and FIG. 7B is a picture ofa printed Bulbasaur. The objects were washed with isopropanol (IPA)after printing.

Example 4

Another composition, C1, was produced using an alkenyl-derivatizedpolysiloxane that included vinyl functional Q resin, vinyl functional Mresin and high molecular weight vinyl terminated silicone. Thealkenyl-derivatized polysiloxane included 30% VQM resin and 70% of vinylterminated silicone. This component is available from Gelest as VQM-135.The amount of mercapto-derivatized polysiloxane was set at a thiol:eneratio of 30:70 and HiSil 900 was used as an additive. In the samplesshown in Table 2, DMS-V25 was added to bring the alkenyl-derivatizedpolysiloxane to 70% for each sample. For example, sample C1 was 70%DMS-V25 and sample C6 was 10% DMS-V25. The HiSil 900 is added at listedquantities of per hundred rubber (PHR) meaning that, for example, sampleC5 included 5 parts HiSil 900 added to 100 parts resin. Results showincreasing viscosity as well as hardness with higher amounts of VQMmaterial. There is also an increase in hardness and viscosity withadditional amounts of silica.

TABLE 2 HiSil 900 Hardness Viscosity ID # VQM-135% PHR (A) (qualitative)Printable? C1 0 0 8 Low Yes C2 10 0 15 Low Yes C3 35 0 18 Moderate YesC4 60 0 21 Moderate Yes C5 35 5 24 Moderate Yes C6 60 10 29.5 Paste Didnot test

Example 5

Composition D1 was produced using the components in Table 3 and wasprinted in a cube at 70 mJ/cm². Results were preferred to those fromcomposition C1 in that the surfaces and corners were noticeablyimproved. A comparison of a cube printed with D1 (left) is compared to acube printed with C1 (right) in FIG. 8 . The photograph illustrates thatthe edges of the D1 material are sharper and the surfaces more planar.

TABLE 3 Component Amount DMS-V25 35% VQM-135 35% SMS-042 30% PhosphineOxide Photoinitiator 2.5 phr Optical Brightener 0.05 phr HiSil 900 5 phr

Additional objects were printed with D1. FIG. 9 illustrates a printedfrog, as printed (FIG. 9A), and as cured at room temperature (FIGS. 9Band 9C). FIGS. 10A-10C show lattices that were printed under the sameconditions as above using composition D1.

Example 6

Three different compositions were tested to evaluate the effect ofdifferent types of mercapto-derivatized polysiloxanes in the physicalproperties of the printed object. In each composition the samealkenyl-derivatized polysiloxane was used at 90% by weight of thecomposition. The specific mercapto-derivatized polysiloxane component ineach composition was varied but the concentration was kept constant at10% by weight. The first example, E1, included a mercapto-derivatizedsilicone homopolymer. The second, E2, was a mercaptosilicone/methylsilicone copolymer. The third, E3, was a branched mercaptosilicone of QTtype. Each sample was printed using the Form3 SLA printer and sampleswere analyzed for hardness, tensile strength and elongation at break.Tensile strength and elongation at break were measured using ASTM D412with type C samples.

TABLE 4 Minimum Mercapto- EnergyDosage Sample derivatized HardnessTensile Elongation to Print ID polysiloxane (Shore A) Strength(MPa) atBreak (%) (mJ/cm²) E1 mercaptosilicone 50 3.6 105 20 homopolymer E2mercaptosilicone 39 1.6 62 35 methyl silicone copolymer E3 QT resin type58 6.1 145 10 mercaptosilicone

These results indicate that the use of a branched mercapto-derivatizedpolysiloxane in SLA printing provides a printed silicone object thatexhibits greater hardness, improved tensile strength and greaterelongation at break when compared to the same composition that uses thetraditional mercapto-derivatized silicone homopolymer ormercapto-derivatized silicone methyl silicone copolymer. Thisimprovement is realized at the same ratio of alkenyl-derivatizedpolysiloxane to mercapto-derivatized polysiloxane and also at the sameor a similar ratio of thiol groups to alkene bonds. The results alsoshow that well-defined silicone objects can be printed at energy dosagesof less than 35, less than 20 or less than or equal to 10 mJ/cm². Theability to print at lower energy dosages means that printing can beperformed more quickly and that parts can be produced in a fraction ofthe time necessary for methods requiring higher energy dosages to effectcure.

Example 7

Brittling behavior of the cured composition was examined. Additives ofhindered amines were found to reduced brittling over a three month timeperiod, as shown in FIG. 11 .

Such alterations, modifications, and improvements are intended to bepart of this disclosure and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and figures are byway of example only.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the figures.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Further, some actions are described as taken by a “user.” It should beappreciated that a “user” need not be a single individual, and that insome embodiments, actions attributable to a “user” may be performed by ateam of individuals and/or an individual in combination withcomputer-assisted tools or other mechanisms.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

The terms “approximately” and “about” may be used to mean within ±20% ofa target value in some embodiments, within ±10% of a target value insome embodiments, within ±5% of a target value in some embodiments, andyet within ±2% of a target value in some embodiments. The terms“approximately” and “about” may include the target value. The term“substantially equal” may be used to refer to values that are within±20% of one another in some embodiments, within ±10% of one another insome embodiments, within ±5% of one another in some embodiments, and yetwithin ±2% of one another in some embodiments.

The term “substantially” may be used to refer to values that are within±20% of a comparative measure in some embodiments, within ±10% in someembodiments, within ±5% in some embodiments, and yet within ±2% in someembodiments.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. An additive method of producing athree-dimensional (3D) object, the method comprising: providing aphotocurable silicone composition in a vessel of an additive fabricationdevice; irradiating a layer of the photocurable silicone compositionwith an energy source to form an at least partially cured layer of thephotocurable silicone composition on a build platform of the additivefabrication device or on a previously cured layer of the photocurablesilicone composition; wherein the photocurable silicone compositioncomprises: a mercapto-derivatized polysiloxane having two or morefunctional groups; an alkenyl-derivatized polysiloxane; and aphoto-initiator.
 2. The method of claim 1, wherein the photocuredsilicone composition can have a ratio of mercapto-derivatizedpolysiloxane to alkenyl-derivatized polysiloxane by weight of greaterthan 1:99, greater than 5:95, greater than 10:90, greater than 20:80, orgreater than 30:70.
 3. The method of claim 1, wherein the photocuredsilicone composition can have a ratio of mercapto-derivatizedpolysiloxane to alkenyl-derivatized polysiloxane by weight of less than50:50, less than 40:60, or less than 30:70.
 4. The method of claim 1,wherein the alkenyl-derivatized polysiloxane has an average of at leasttwo silicon-bonded ethylenically unsaturated groups per molecule.
 5. Themethod of claim 1, wherein the photocurable silicone composition canhave a ratio of sulfur atoms to vinyl groups of between 1:1 and 3:1, orbetween 1.5:1 and 2.5:1, or between 1.5:1 and 5:1, or between 2.5:1 and5:1.
 6. The method of claim 1, wherein the photocurable siliconecomposition comprises a SiO₄ resin, a CH₃SiO₃ resin, or a (CH₃)₃SiOresin, or a combination thereof.
 7. The method of claim 1, wherein thephotocurable silicone composition comprises a functionalized silica,alumina, fumed silica, or carbon nanotubes, or combinations thereof. 8.The method of claim 1, wherein the energy source is a UV laser.
 9. Themethod of claim 1, wherein the energy source is a UV laser delivering adosage of greater than 10 mJ/cm², greater than 20 mJ/cm², greater than30 mJ/cm², greater than 40 mJ/cm², greater than 50 mJ/cm², greater than60 mJ/cm², greater than 65 mJ/cm², or greater than 80 mJ/cm².
 10. Themethod of claim 1, wherein the mercapto-derivatized polysiloxane has aviscosity of greater than 100 cP.
 11. The method of claim 1, wherein thealkenyl derivatized siloxane has a viscosity of greater than 400 cP. 12.The method of claim 1, wherein the energy source is selected from atleast one of UV laser, UV LCD or UV DLP.
 13. The method of claim 1,wherein multiple at least partially cured layers in the shape of athree-dimensional object are post-cured at a temperature between roomtemperature and 125° C., less than 120° C., less than 110° C., less than100° C., less than 90° C., less than 80° C., less than 75° C., less than70° C., less than 65° C., or less than 60° C.
 14. The method of claim 1,wherein multiple at least partially cured layers in the shape of athree-dimensional object are post-cured at greater than 55° C. for atleast 10 minutes.
 15. The method of claim 1, wherein the photocurablesilicone composition has a viscosity of greater than 500 cP, greaterthan 600 cP, greater than 700 cP, greater than 1000 cP, greater than2000 cP, greater than 5,000 cP, greater than 10,000 cP, or greater than15,000 cP.
 16. The method of claim 1, wherein the photocurable siliconecomposition has a viscosity of less than 500,000 cP, less than 400,000cP, less than 300,000 cP, less than 200,000 cP, less than 100,000 cP,less than 50,000 cP, less than 25,000 cP, less than 20,000 cP, less than15,000 cP, less than 10,000 cP, less than 5,000 cP, less than 4,000 cP,less than 3,000 cP, less than 2,000 cP, less than 1000 cP, less than 800cP, less than 500 cP, or less than 300 cP.
 17. The method of claim 1,wherein the mercapto-derivatized siloxane comprises a QT resin, a QMresin, a QDT resin, a D resin, a QTM resin, a QTDM resin, or acombination thereof.
 18. The method of claim 1, wherein thealkenyl-derivatized siloxane comprises a QT resin, a QM resin, a QDTresin, a D resin, a QTM resin, a QTDM resin, or a combination thereof.19. The method of claim 1, wherein the mercapto siloxane comprisespropylthiol groups.
 20. The method of claim 19, wherein themercapto-derivatized siloxane is essentially devoid of methyl groups.21. The method of claim 1, wherein the mercapto-derivatized siloxane hasan SH functionality of greater than 4%, greater than 8%, greater than12%, or greater than 14%.
 22. The method of claim 1, wherein thecomposition comprises less than 5%, less than 1%, less than 0.1%, orless than 0.01% silicone monomer, by weight.
 23. The method of claim 1,wherein the photocurable silicone composition comprises a blend of twoor more distinct mercapto-derivatized polysiloxanes.
 24. The method ofclaim 1, wherein the photocurable silicone composition comprises a blendof two or more distinct alkenyl-derivatized polysiloxanes.
 25. Themethod of claim 1, wherein the mercapto siloxane comprises a resinwherein some or all of the organic substituents are alkylthiol groups.26. The method of claim 1, wherein the alkenyl-derivatized polysiloxaneincludes a plurality of vinyl groups.
 27. The method of claim 1, whereinthe alkenyl-derivatized polysiloxane includes a plurality of acrylategroups.
 28. The method of claim 1, wherein the photocurable siliconecomposition comprises a flame retardant.
 29. The method of claim 1,wherein the photocurable silicone composition comprises an odorneutralization agent.
 30. The method of claim 1, wherein thephotocurable silicone composition comprises a scent masking agent. 31.The method of claim 1, wherein providing the photocurable siliconecomposition includes extruding the photocurable silicone composition.32. The method of claim 1, wherein providing the photocurable siliconecomposition includes providing a bath of the photocurable siliconecomposition.